Field of the Invention
The present invention relates to a method and apparatus for magnetic resonance imaging, and particularly, to a method and apparatus for magnetic resonance imaging that make use of a spectrum suppression pulse.
Description of the Prior Art
Magnetic resonance imaging is a bio-magnetic nuclear spin imaging technology developed rapidly along with the development of computed technology, electronic circuit technology, and superconductor technology. In magnetic resonance imaging, human tissue is placed in a static magnetic field B0, and hydrogen nuclei within the human tissue are then excited by a radio-frequency pulse with the same frequency as the precession frequency of the hydrogen nuclei, so as to cause resonance of the hydrogen nuclei (nuclear spins) and absorption of energy, which causes the nucleus spins to be deflected out of alignment with the static field by an amount known as “flip angle.” After the radio-frequency pulse ceases, the hydrogen nuclei emit a radio signal at a specific frequency and release the absorbed energy, which is detected as a radio-frequency signal that is processed by a computer to obtain an image in magnetic resonance imaging.
In order to obtain an image of better quality, signals having a specific spectral composition, such as fat signals, water signals, silica gel signals (breast implants) and so on, often need to be suppressed. For example, in magnetic resonance imaging examinations used for the abdomen, the chest, etc., it is generally required to suppress fat signals, so as to display tissues or lesions of interest in a highlighted way. Many fat suppression techniques have been proposed, such as the FatSat (fat saturation) technique, SPAIR (Spectral Presaturation Attenuated Inversion Recovery) technique, STIR (short inversion time inversion recovery) technique, Dixon water-fat separation technique, and so on. Except for the Dixon technique, the efficacy of all the other techniques is dependent on the flip angle (such as for FatSat) or inversion time (such as for inversion recovery techniques like SPAIR, STIR, etc.) of a fat suppression pulse applied to the human tissue. It is well known that a flip angle refers to the angle of a macro magnetization vector deviating from the static magnetic field B0 under the excitation of a radio-frequency pulse. The size of the flip angle is determined by the intensity and action time of the applied radio-frequency pulse. Inversion time is the time interval between a 180 degree inversion pulse and an excitation pulse in an inversion recovery pulse sequence.
As described above, optimization of fat suppression can be achieved by optimizing the flip angle or inversion time of the fat suppression pulse. In response to this situation, a method for calculating a flip angle of a FatSat pulse has now been proposed. However, this method only takes into consideration the calculation method of the flip angle of the FatSat pulse when a spin echo sequence is used for magnetic resonance imaging, without taking into account a number of excitation pulses applied following each fat suppression pulse in an abdomen magnetic resonance imaging applying a spoiled gradient echo sequence (Spoiled GRE, such as VIBE sequence) of a short radio-frequency pulse interval (TR). Furthermore, the method cannot be used to optimize the inversion time of the fat suppression pulse (such as for inversion recovery pulses like SPAIR, STIR, etc.).